It is widely accepted that the level of activity alters the biology and biomechanics of normal, repaired, and reconstructed ligaments. Significant research resources are directed at determining the relation between in vivo forces and biological response. In order to design critical in vivo experiments it is necessary to know which activities cause the ligaments to become loaded, and the factors which control the magnitude of the ligament forces. Considering the importance of knowledge about in vivo ligament forces it is surprising how little is known. Such knowledge would impact the design of ligament substitutes, rehabilitation following soft tissue injury and reconstruction, and our understanding of the mechanism controlling homeostasis, healing, and remodeling of ligaments and grafts. In this study we examine associations between in vivo ligament forces and five variables; joint position, quadriceps force, speed of locomotion, and anterior limb acceleration. Experiments will provide data on forces in two ligaments, the anterior cruciate and medial collateral, and in the patellar tendon. In addition we will measure the acceleration of the tibia along its anterior direction, and the rotational motions of the tibio-femoral joint. These variables will be measured during walking with a slow, natural, and fast cadence, trotting, stopping and starting, turning, lying down and rising up, stepping up and down a rise, and kicking. Each activity will be repeated at least five times. Analysis of variance and correlation techniques will be used to test the hypotheses. A unique feature of this research is the use and further development of a new device for measuring ligament and tendon forces in vivo. The new device is implanted within the substance of the tissue, and responds to the lateral compressive stress produced by the axial stresses the collagen fibers whose trajectory has been displaced by the device.